1. Field of the Invention
The present invention generally pertains to nickel-metal hydride batteries and, more particularly to methods and apparatus to recharge such batteries.
2. Description of Related Art
A Nickel-Metal Hydride (hereafter NiMH) battery has a State-of-Charge (SOC) associated therewith. Generally speaking, there are two types of SOC: discharging SOC and charging SOC. The discharging and charging SOC are related such that 100% discharging SOC is equal to the charging SOC needed to fully charge the battery (conventionally 110%-120%). The SOC is related to the battery capacity, such that a fully charged battery has 100% discharging SOC. When a NiMH battery is repeatedly undercharged (&lt;105% charging SOC), its voltage tends to be temporarily reduced. In this regard, undercharging can be either caused manually by the user (premature ending of charging operation and subsequent discharge) or by high temperature operation (automatic termination of charging operation due to high temperature conditions). Undercharging and resulting voltage reduction leads to a reduction in battery capacity, otherwise known as the "memory effect".
Memory effect is the temporary under-voltage state a NiMH battery is under when repeatedly under-charged or not completely discharged. The memory effect reduces the battery capacity, and can result in a shorter range and slower acceleration in an electric vehicle, or reduced run-time in a cellular phone, portable computer, power-tool, or video camera. In addition, the memory effect makes it difficult for a battery management system to distinguish between an actual deteriorated battery and a battery with memory effect.
In response to the problems posed by the "memory effect", a recharging method wherein a high charging SOC is applied, illustrated in FIG. 1, has become conventional. Unfortunately, as will be discussed more fully hereafter, the conventional solution to the "memory effect" problem, while theoretically appealing, has several real-world drawbacks that limit its useful application.
As is shown schematically in FIG. 1, during each recharge cycle a charging SOC of between about 110% to 120% SOC is applied to the battery. The recharging operation is terminated when the preset SOC limit has been reached. The two most common methods for determining the end of charge (EOC) or charging SOC limit of a NiMH battery are a negative change in voltage (-dV) or a time-based temperature gradient (dT/dt). Setting the charging SOC limit in excess of 105% SOC theoretically ensures maximum discharge capacity from the battery and reduces the memory effect.
However, when a NiMH battery is being charged in accordance with the conventional method illustrated in FIG. 1, heat is produced as a result of exothermic reactions, joule losses, and pressure build-up through oxygen evolution of the positive electrode. High temperature operation severely degrades the cycle life of NiMH batteries, and limits their useful life.
In order to fully practice the aforementioned conventional method, it may be necessary to actively cool the NiMH battery while recharging same. However, development of a low cost, light weight, efficient cooling system for a NiMH battery pack has proven difficult. Such a cooling system would be especially desirable for NiMH batteries used in electric vehicles, most notably when such vehicles are used in hot areas or during the summer months.
Accordingly, recharging in accordance with the conventional method illustrated in FIG. 1 will lead to heat generation and shortened battery life. A low SOC setting (less than 105% charging SOC) for recharging will reduce battery capacity and cause the memory effect.
Accordingly, there is a need in the art for a method and a device for effectively recharging a NiMH battery wherein the potential for damaging heat generation and memory effect is reduced.